THE LEUCOCYTE 



1897 



The Leucocyte. 



In post-embryonic life the home of the leucocyte or colourless 

 blood cell is the bone-marrow. They may be divided into the 

 hyahne cells or lymphocytes, which are the least differentiated 

 cells, both ontogenetically and phylogenetically, being the first 

 to appear in the embryo and in the lower animals, and the granular 

 cells or leucocyte sensu stricto, which are more highly evolved cells. 



It is usual to consider the large lymphocytes, the small lympho- 

 cytes, the large mononuclears, and the transitional cells, as all 

 belonging to the same denomination. According to Heidenhain, 

 there is a tendency on the part of the centrosome to reach the centre 

 of the lymphocyte, being pulled thereto by cytoplasmic radii. 

 It is, however, prevented from assuming this position by the nucleus, 

 which succeeds in this obstruction in the case of the small and 

 large lymphocyte, because there is insufficient cytoplasm; but in the 

 case of the large mononuclear it pushes the nucleus to one side, and 

 in the case of the transitional it so indents the nucleus that it attains 

 in both cases the central position. Thus the small lymphocyte b}^ 

 growth can become the large lymphocyte, from which are descended 

 the large mononuclear and the transitional, which may, therefore, 

 be reckoned as a single type in making a differential count. 



The small lymphocyte is about the size of an erythrocyte, but 

 somewhat smaller; it possesses a rounded or slightly indented 

 nucleus, colouring deeply with basic stains, and surrounded by a 

 very slight amount of cytoplasm, which shows a reticulum with 

 basophile granules as the nodal points. There are also some 

 azurophile granules. 



The large lymphocyte is larger, reaching to 20 microns, and is 

 characterized by a central roundish or slightly indented nucleus, 

 which contains less chromatin than in the small variety. There is 

 more abundant cytoplasm, which does not stain so deeply, but 

 contains the same reticulum and Wolff's azurophile granules. 



The large mononuclear leucocyte possesses an excentric nucleus 

 relatively poor in chromatin, surrounded by a larger amount of 

 cytoplasm, with the usual reticulum and azurophile granules. 



The transitional cell has its nucleus indented into a horseshoe 

 or sometimes twisted. Lymphocytes and myeloblasts may contain 

 fuchsinophile granules called Schridde's granules. The true leuco- 

 cytes or granular white cells are classified according to their 

 granules into neutrophile, eosinophile, and basophile. Their parents 

 are the myelocytes of the bone-marrow, which are cells of large size 

 possessing rounded pale-staining nuclei and the granules typical of 

 the particular leucocyte to which they are to give rise. 



The most primitive form is the eosinophile leucocyte which is so 

 common in lower animals. It probably has a secondary home in 

 connective and lymphoid tissue. They are characterized by their 

 large refractile granules, which have an affinity for acid stains and a 

 twisted, trilobed, or dumb-bell-shaped nucleus. 



